1. Field of the Invention
The present invention relates to a digital interface between at least two subscriber line interface circuits for the connection of analog telephone subscriber lines of a digital time-multiplexed telecommunications network and a processing unit performing the coding or decoding and filtering of the telephone data transmitted over the subscriber lines under the control of its own processor, and which also provides the instructions to set up various operating conditions of the subscriber line interface circuits and the instructions which determine the subscriber line status indications initiated by the subscriber line interface circuits.
2. Description of the Prior Art
It is well known that with the aid of integrated circuit technology, the space requirements for relatively complex electrical units have been constantly reduced. A factor that will probably limit this continuing trend is the number of plugs required for such an electrical unit, which, in view of connections that must be made, such as by soldering, cannot be arranged as close to one another as are desirable.
Until now, serial communication interfaces have been provided between integrated subscriber line interface circuits and processing units of the aforementioned type, so that connections have various types of signals coupled thereto or are used to transmit various types of signals, such that the total number of required connections is relatively small in proportion to the number of the various types of signals to be received or transmitted. This, however, means having to accept a longer time to transmit the signals. Additionally, there is an increased circuit expenditure to manage control of such a known serial interface.
In practice, two types of subscriber line interface circuits are applicable. One such interface circuit is the ER-SLIC (External Ringing Subscriber Line Interface Circuit) whose important characteristic differentiating it from the other type of interface circuit is that its ringing current pulses are generated outside of the unit and a relay is used for their feeding in and control. The other type of subscriber line unit is the IR-SLIC (Internal Ringing Subscriber Line Interface Circuit) wherein ringing current and monitoring pulses are derived internally from the supply voltage. One solution for such an IR-SLIC provides an input/output register.
The primary object of the present invention is to provide a digital interface of which is a universal interface for the connection of both types of subscriber line interface circuits and which has a relatively small number of connector plugs, without having the disadvantages of a serial interface.